A pursuit spacecraft from the planet Tatooine is attempting to catch up with a Trade Federation cruiser. As measured by an observer on Tatooine, the cruiser is traveling away from the planet with a speed of 0.6c. The pursuit ship is traveling at a speed of 0.8c relative to Tatooine, in the same direction as the cruiser. What is the speed of the pursuit ship relative to the cruiser?

Aug 18th 2012, 05:23 PM

HallsofIvy

Re: Special relativity question

I believe I answered this on another board and said that I agreed with the result you gave their. But perhaps we were both looking at this incorrectly (I'm not a physicist!). We both were doing this as . But relative the cruiser, Tatoine has a velocity of -.8c. So the "sum" of speeds should be .

Aug 18th 2012, 05:34 PM

GJA

Re: Special relativity question

Hey, guys.

I think the numerator should be reversed:

is the velocity of sprinter ship relative to Tatooine and

is the velocity of Tatooine relative to the cruiser ship.

Aug 18th 2012, 05:41 PM

HallsofIvy

Re: Special relativity question

If that were the case, the sprinter ship would see the cruiser ship with speed +.38c and it would not be gaining!

Aug 18th 2012, 06:45 PM

GJA

Re: Special relativity question

The velocity is measured relative to the cruiser, so it should be gaining.

Aug 19th 2012, 03:00 AM

alexmahone

Re: Special relativity question

Quote:

Originally Posted by HallsofIvy

I believe I answered this on another board and said that I agreed with the result you gave their. But perhaps we were both looking at this incorrectly (I'm not a physicist!). We both were doing this as . But relative the cruiser, Tatoine has a velocity of -.8c. So the "sum" of speeds should be .